Lysosomal storage diseases (LSDs) result from the deficiency of specific lysosomal enzymes within the cell that are essential for the degradation of cellular waste in the lysosome. A deficiency of such lysosomal enzymes leads to accumulation within the lysosome of undegraded “storage material,” which causes swelling and malfunction of the lysosomes and ultimately cellular and tissue damage. A large number of lysosomal enzymes have been identified and correlated with their related diseases. Once a missing enzyme has been identified, treatment can be reduced to the sole problem of efficiently delivering a replacement enzyme to the affected tissues of patients.
One way to treat lysosomal storage diseases is by intravenous enzyme replacement therapy (ERT) (Kakkis, Expert Opin. Investig. Drugs 11(5): 675-685, 2002). ERT takes advantage of the vasculature to carry enzyme from a single site of administration to most tissues. Once the enzyme has been widely distributed, it must be taken up into cells. The basis for uptake into cells is found in a unique feature of lysosomal enzymes. Lysosomal enzymes constitute a separate class of glycoproteins defined by phosphate at the 6-position of terminal mannose residues. Mannose-6-phosphate is bound with high affinity and specificity by a receptor found on the surface of most cells (Munier-Lehmann et al., Biochem. Soc. Trans. 24(1): 133-136, 1996; Marnell et al., J. Cell. Biol. 99(6): 1907-1916, 1984). The mannose-6-phosphate receptor (MPR), which has two mannose-6-phosphate binding sites per polypeptides chain (Tong et al., J. Biol. Chem. 264:7962-7969, 1989), directs uptake of enzyme from blood to tissue and then mediates intracellular routing to the lysosome.
Large-scale production of lysosomal enzymes involves expression in mammalian cell lines. The goal is the predominant secretion of recombinant enzyme into the surrounding growth medium for harvest and processing downstream. In an ideal system for the large-scale production of lysosomal enzymes, enzyme would be efficiently phosphorylated and then directed primarily toward the cell surface (i.e., for secretion), rather than primarily to the lysosome. As described above, this partitioning of phosphorylated lysosomal enzymes is the exact opposite of what occurs in normal cells. Manufacturing cell lines used for lysosomal enzyme production focuses on maximizing the level of mannose-6-phosphate per mole of enzyme, but is characterized by low specific productivity. In vitro attempts at producing lysosomal enzymes containing high levels of mannose-6-phosphate moieties have resulted in mixed success (Canfield et al., U.S. Pat. No. 6,537,785). The in vitro enzyme exhibits high levels of mannose-6-phosphate, as well as high levels of unmodified terminal mannose. Competition between the mannose-6-phosphate and mannose receptors for lysosomal enzyme results in the necessity for high doses of enzyme for effectiveness, and could lead to greater immunogenicity to the detriment of the subject being treated.
Sulfatases constitute a unique subclass of lysosomal enzymes. Sulfatases cleave sulfate esters from a variety of substrates, including, for example, steroids, carbohydrates, proteoglycans and glycolipids. All known eukaryotic sulfatases contain a cysteine residue at their catalytic site. Sulfatase activity requires post-translational modification of this cysteine residue to Cα-formylglycine (FGly). The cysteine to FGly post-translational enzyme activation occurs within the endoplasmic reticulum on unfolded sulfatases immediately after translation, prior to targeting of the sulfatases to the lysosome (Dierks et al., Proc. Natl. Acad. Sci. USA 94:11963-11968, 1997). The formylglycine-generating enzyme that catalyzes this reaction is sulfatase modifying factor 1 (SUMF1). Highlighting the importance of this unique post-translational modification is the fact that mutations in SUMF1, which result in impaired FGly formation in lysosomal sulfatase enzymes, cause Multiple Sulfatase Deficiency (MSD) in man (Diez-Ruiz et al., Annu. Rev. Genomics Hum. Genet. 6:355-379, 2005).
Accordingly, the therapeutic effectiveness of a lysosomal sulfatase enzyme preparation depends on the level of mannose-6-phosphate, and on the presence of active enzyme, in that preparation.
Thus, there exists a need in the art for an efficient and productive system for the large-scale manufacture of therapeutically effective, active highly phosphorylated lysosomal sulfatase enzymes for management of lysosomal storage disorders caused by or associated with a deficiency of such lysosomal sulfatase enzymes.